Many acoustic sensors include sound-responsive elements that resonate when they are exposed to acoustic energy. Such sound-responsive elements are typically mechanical structures that include any one of the following: a stretched membrane, a clamped diaphragm, a magnetic diaphragm held in place by magnetic forces, conical diaphragms, circular pistons, and corrugated-ribbon conductors. To transform the acoustic energy detected by the aforementioned sound-responsive elements, transducers are coupled to the sound-responsive elements. Typical transducers include the following: loose-contact transducers; moving-iron transducers; electrostatic transducers; piezoelectric transducers; and moving coil transducers.
The transducers and their corresponding sound-responsive elements have inherent size limitations as well as acoustic frequency response limitations. For example, a carbon microphone employs a diaphragm that vibrates in accordance with impulses of sound and then turns the vibrations into electric energy. The vibrations are converted into electrical energy by a piezoelectric transducing method where the diaphragm compresses the carbon granules which in turn changes the electrical resistance of the carbon granules. The electrical resistance of the carbon granules is detected by a voltage measuring device and can be processed by a correlating device which can associate the measured resistance to an acoustic frequency and acoustic intensity. With the voltage measuring device and piezoelectric tranducing method, the operating range for a carbon microphone is typically between 100 and 5,000 hertz (Hz). The carbon microphone is typically used in the mouth piece or transmitter of a telephone handset.
Carbon microphones have a finite length, width, and height that are dependent upon the range of acoustic frequencies that are intended to be detected by the carbon microphone. Due to this physical size dependency on the range of acoustic frequencies intended to be detected, a carbon microphone is typically bulky. Another drawback of a carbon microphone is that it can generate noise. The noise is typically a result of carbon granules that are packed too loosely. Another disadvantage of the carbon microphone is that if an acoustic frequency of interest lies outside the operating range of the carbon microphone, the actual physical dimensions of the carbon microphone and more specifically the sound-responsive element and transducers of the carbon microphone, must be changed or resized in order to detect such a frequency,
Unlike the transducers of the carbon microphone, a dynamic microphone includes a moving coil transducer to convert the movement of a pole into electrical energy. The pole is attached to a diaphragm and is moved by sound waves striking the diaphragm. A dynamic microphone typically includes many parts: a large base permanent magnet, a coil, a diaphragm held in place by a ring, a washer, and large air spaces disposed within the permanent magnet. The dynamic microphone typically has an acoustic frequency response of 30 Hz to 18 kHz. While the dynamic microphone may a large acoustic frequent response range relative to the carbon microphone, the number and size of the dynamic microphone's operating components render it impractical for use in acoustic arrays and other array like applications. Similar to the carbon microphone, the dynamic microphone is typically designed for a set or fixed acoustic frequency range and therefore, requires substantial mechanical retrofitting or resizing to alter its acoustic frequency range response.
In addition to the size and frequency response limitations of both the dynamic microphone and carbon microphone, these microphones can cause mutual interference in large array applications. For example, if several carbon or dynamic microphones having the same frequency range are placed adjacent to one another, neighboring microphones may interfere with one another due to the sound-responsive elements of each of these microphones being tuned to the same acoustic frequency. Vibrations of one sound responsive element of a microphone may generate acoustic energy or noise relative to a neighboring microphone having a similar size sound-responsive element.
Thus there is a need for an acoustic sensor which avoids the drawbacks of the size and sensitivity of conventional acoustical sensors. There is further need for an acoustic sensor having reduced number of sound-responsive elements. Another need exists for an acoustic sensor that can reduce mutual interference between acoustic sensors placed in an array.